sign-file: Fix warning about BIO_reset() return value

Fix the following warning:

	scripts/sign-file.c: In function ‘main’:
	scripts/sign-file.c:188: warning: value computed is not used

whereby the result of BIO_ctrl() is cast inside of BIO_reset() to an
integer of a different size - which we're not checking but probably should.

Reported-by: James Morris <jmorris@namei.org>
Signed-off-by: David Howells <dhowells@redhat.com>